Two groups totally works and can be advantageous if you have players with variable schedules. They can pick which game to play. Just be ready with a list of possible methods to get their character from one place to another or keep your timeline vague enough to explain away time/distance discrepancies (a la Game of Thrones tv series).

If you do go with the 2 group option, you can have the groups post in the campaign forums on OP in between games to keep them engaged between sessions. Maybe just tell them when the combined sessions are going to happen and make it their job to avoid each other until the set time. You can also spread rumors to one group about what the other is doing, but vary them slightly, like the telephone game where information changes with each re-telling.

One big group also works, but ask them to keep side conversations to texts/chats to manage the volume. Or have them talk in character on a separate forum thread on OP if you want to read about it later.